Testing the resistance of an unknown resistor can be performed in a number of ways. Typically, it involves applying a known voltage across the resistance and measuring the current flow through the resistor, or driving a known current through the resistor and measuring the voltage differential across the resistance. Driving a known current is a commonly preferred technique due to accuracy of current sources and voltmeters in comparison to voltage sources and ammeters.
In conventional testing systems, testing is accomplished by putting an unknown resistance in parallel with a known resistance value. It is common for both the known and unknown resistances to be connected to a common ground. A known current is driven through the circuit, and a voltage reading across the parallel resistance is taken. A simple Ohm's law calculation can be made to determine the resistance of the overall parallel circuit. The parallel combination of two resistances can then be examined to determine the resistance of the unknown resistance. One skilled in the art will appreciate that though the discussion thus far has centered on resistance determinations, the impedance of an element can be determined in the complex plane using the same technique.
Measuring the voltage drop across an element after applying a known current is a preferred technique due to the accuracy of volt meters in comparison to the accuracy of ammeters. To provide high precision measurement systems, the voltmeter must be used in a range that provides a great deal of accuracy. To ensure that the measured voltage is in this region, a large current may be required. For low value resistances, a high driving current is required to prevent the voltmeter from providing results with high percentage errors.
The use of high current sources in measurement systems creates several difficulties, especially where it is important for the power source to be able to reverse the current direction at fixed intervals.
Conventional implementations of high current resistance measurement systems have relied on the use of electro-mechanically and pneumatically actuated relay contacts to reverse the direct current through the resistance device being measured. Commercial power supplies with separate controllers are also used so that the current levels can be measured.
Reversal of the current is employed to mitigate thermal voltage errors that are commonly associated with precision measurement of voltage levels of single digit and lower voltage values. The reversal of the current through the device cancels the thermal voltage error in the measurement process. Because of a requirement to provide very precise measurements, currents are applied that result in measured values on the order of a few hundred millivolts or lower. Current reversal can also be employed to allow equipment to undergo a self calibration routine.
Reversing a current source that is generating a low current is comparatively easy to do, while reversal of a large current source typically involves the use of physical relay switches. The actuation of the relay contacts in conventional systems results in inefficiencies in both space and power consumption. Separate means are also required to operate the contacts, and the contact surfaces must be regularly maintained and inspected to prevent damage due to both mechanical contact and electrical arcing.
Conventional high current reversal systems make use of compressed air actuated plungers and large contact surfaces. This allows for control of the direction of current, and minimizes the resistance across the relay contacts. This requires four independent contact pairs so that current reversal and interconnection of the contacts between the power supply and the measurement instrument is provided.
The use of current comparator technology has been used extensively in the measurement of resistances to high accuracy levels of better than 1 part per million and currents above 3000 amperes. The problems associated with generating and reversing currents above a few hundred amperes were solved with the use of mechanical compressed air operated relay contacts and the combining of multiple commercial power supplies to achieve the necessary current levels. The further commercial exploitation of the technology at the higher current levels has been limited due to the high cost of implementation.
The large physical switches introduce mechanical wear as a source of failure, increase the required maintenance regime, dissipate unnecessary energy, and as they age, become increasingly unreliable without a dedicated maintenance regime.
It is, therefore, desirable to provide a high current resistance measuring system that does not rely upon mechanical switching.